But if you aren't ready to go to a Mac, I would start by optimizing your computer and possibly adding additional RAM (at least 1GB, but preferably 2GB total).

Be sure to get any service updates Microsoft issues, and then make sure your virus software is up-to-date. Then download a program called "Spybot Search & Destroy". You will need to run that program every 2-3 days to make sure that all of your spyware is removed. Spyware and Trojans are the most likely cause of your problems. The problem is that even some of the best programs cannot remove all of them.

In the end, you may just have to format your hard drive and start from scratch. It was this annual format and start over ritual that finally led me to switching to Mac, and its the best decision I've ever made regarding computers.